Solar Cells Made Obsolete

PORTLAND, Ore.--Now before you get all excited by the headline, which is not click-bait according to the researchers, a new kind of nanoscale rectenna (half antenna and half rectifier) can convert solar and infrared into electricity, plus be tuned to nearly any other frequency as a detector. The invention was made at Georgia Tech (Atlanta) and peer-reviewed in today's issue of Nature Nanotechnology.

Right now efficiency is only one percent, but in the paper (DOI: 10.1038/nnano.2015.220) professor Baratunde Cola and colleagues at the Georgia Institute of Technology (Georgia Tech, Atlanta) convincingly argue that they can achieve 40 percent broad spectrum efficiency (double that of silicon and more even than multi-junction gallium arsenide) at a one-tenth of the cost of conventional solar cells (and with an upper limit of 90 percent efficiency for single wavelength conversion).

video: https://youtu.be/kMdYVLtBtoYProfessor Baratunde Cola explains and demonstrates how optical rectennas are going to change the world by making cheap solar-to-electricity converters out of carbon nanotubes whose end had been turned into a metal-insulators-metal tunnel diode.
(Source: Georgia Tech, used with permission)

Based on a theory put forth over 50-years ago the rectenna idea was before its time, but now technology has finally caught-up to the concept, which is well suited for mass production, according to Cola. It works by growing fields of carbon nanotubes vertically, the length of which roughly matches the wavelength of the energy source (one micron for solar), capping the carbon nanotubes with an insulating dielectric (aluminum oxide on the tethered end of the nanotube bundles), then growing a low-work function metal (calcium/aluminum) on the dielectric and voila--a rectenna with a two electron-volt potential that collects sunlight and converts it to direct current (DC).

Schematic of the components making up the optical rectenna--carbon nanotubes capped with a metal-oxide-metal tunneling diode. (Credit: Thomas Bougher)
(Source: Georgia Tech, used with permission)

"Our process uses three simple steps: grow a large array of nanotube bundles vertically; coat one end with dielectric; then deposit another layer of metal," Cola told EE Times. "In effect we are using one end of the nanotube as a part of a super-fast metal-insulator-metal tunnel diode, making mass production potentially very inexpensive up to 10-times cheaper than crystalline silicon cells."

The structure resembles a metal-insulator-metal capacitor of a few attofarads, but by reducing area of the capacitor "plate" (each nanotube bundle is just 10-to-20 microns in diameter), the high electrical field concentration at the end of the nanotube, and the low work function of the metal, makes the device behave like a peta-hertz tunnel diode excited by solar energy emitting electrons in femtosecond bursts.

For Cola's proof of concept, his group grew multi-nanotube bundles of about 8-to-10 microns in diameter, which they then scaled up to thousands of side-by-side bundles measuring about an inch square which produced microwatts of output power from the sun.

For commercialization, billions or even trillions of carbon-nanotube bundles could be grown side-by-side, ramping up the power output into the megaWatt range, after optimization for higher efficiency.

"We still have a lot of work to do to lower contact resistance which will improve the impedance match between the antenna and diode, thus raising efficiency," Cola told us."Our proof-of-concept was tuned to the near-infrared. We used infrared-, solar- and green laser-light and got efficiencies of less than one percent, but what was key to our demo was we showed our computer model matched our experimental results, giving us the confidence that we can improve the efficiency up to 40 percent in just a few years."

For the future, Cola's group has a three tiered goal--first develop sensor applications that don't require high efficiencies, second to get the efficiency to 20 percent for harvesting waste heat in the infrared spectrum, then start replacing standard solar cells with 40 percent efficient panels in the visible spectrum. The team is also seeking suitable flexible substrates for applications that require bending.

"Now we are at a point where engineers in industry, can reach out and give us more application conditions," Cola told us. "One thing we can do that semiconductors can't is operate at ultra-high temperatures--they never have to be cooled to work well. In fact, they work better at temperatures up to 400-to-500 degrees Celsius."

Cola estimates that 10-to-20 research groups worldwide are working on retennas, but his is the only one to change the paradigm to make vertical instead of planar devices, which makes all the difference.

Other researchers contributing to the work included Asha Sharma, Virendra Singh and Thomas Bougher.

Funding was provided by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO).

Yes, the existig solar cell makers will be doing everything they can to improve the efficiency of their panels, both to beat each other and to be a moving target to upstart technologies which sometimes take decades to perfect.

Many who know these researchers say that if it can be done, they will reach 40% and beyond, but of course not all research pans out as planned--and is sometimes bought up by competitors who intentionally kill the project--patents and all--just to protect their investments.

Yes, you are right. 90% or maybe even 99% of "breakthroughs" run into some kind of insurmountable hurdle, or are just bought up and killed by a competitor protecting his own turf. Unfortunately, there is no way to know ahead of time which will fail and which will change-the-world (for the better). That's why its great that government grants a willing to take chances, in order to sift through and find the winning needle in the haystack.

When SolarCity brings its plant in South Buffalo online next year, their panels will exhibit a conversion efficiency of at least 21%. With further design optimization they claim they will be at 24% conversion efficiency not too long after.

The title of this article is misleading. Given that the broadband rectenna efficiency limit is 44%, it would be quite difficult to reach 90% conversion efficiency using the sun, which is not monochromatic, as the power source. It would be quite amazing if they were able to reach even the 40% that they claim. I wonder if they intend to use spectral splitting to achieve this very ambitious goal?

This is exciting technology, I hope it truly can reach 40% efficiency. Heck even 30% would be fantastic if it is truly low cost to produce. I agree with the judicious use of federal funds for basic research, where the cost to get to a proof-of-concept stage is fairly low, it can be money well spent. HOWEVER (I'm climbing on my soapbox now) there is a certain toxic branch of science that has consumed, and continues to do so, a far larger piece of the grant money pie than is reasonable (billions per year), with virtually nothing except scary stories to show for it. It is trashing the reputation of federally funded research, if it is not contained soon it will ruin it for all researchers, and we will be worse off as a nation. The appropriate number is millions, not billions, per year for this branch. You probably know already which branch I am referring to. It is climate research. Please, for the sake of western civilization, help put an end to this gravy train of corruption and greed. If you need proof this is occuring, please start digging for yourself. Above all please don't ignore what is happening. Thanks for reading. Steve Harris, BSEE '92

As many have said - we see 'breakthroughs' on an almost daily basis and then never hear of them again. Nevertheless, many of those 'breakthroughs' morph into something else before getting to market, so we don't necessarily recognise them. This technology looks fascinating to me - partly because it has the potential to make a really practical solar energy converter, but mainly because I wonder if there is the possibility of applying it to nuclear reactors. Could the intense gamma radiation be directly converted into electrical energy? Could the neutrons be used to flouresce some material and then convert the light directly into electrical energy? Anything that could convert the fission products directly into electrical energy rather than into steam which is then run through a turbine to create electricity would give us a very attractive source of power.

Bara is a heck of a researcher, so I am very hopeful that he will be able to follow through with this. I worked with him at Purdue when he was getting his Ph.D. and he was able to plow through a variety of roadblocks. This is one invention that I have a great deal of hope in.

I'm perhaps too jaded but I've seen lots of "game changing" research with "5 year" horizons for commercialization come and go with nary a peep after the initial spurt of positive press. As with many other "breakthroughs" I hope I'm proven wrong on this one. But a cursory google search of 2010 solar related "breakthroughs" shows an amazing menagerie of technologies with promise. However the reality seems to be far off. Indeed we seem to have taken a step backwards. The most efficient panel in 2010 was a Sanyo panel at nearly 17.5% efficiency. Today it's a Kyocera panel at 16% efficiency seeing has how Sanyo was bought out and no longer sells the highly efficient panel. C'est la vie I suppose.

As for government funding being strictly a force for good, well, there's the good and the bad. Solyndra, Crescent Dunes, Cash for Clunkers, etc come to mind for the bad and that's all without trying.

Steve, yes you are right. In addition, it usually doesn't take billions or even millions to fund the most experimental, highest risk, highest payoff research. All the researchers usually need is a few hundred thousand to prove the concept, then non-federal funding usually fills the gap to developing a manufacturable products.